The invention relates to automatic transmissions of motor vehicles. More particularly, the invention relates to an apparatus for controlling the gear changing process in automatic transmissions under special conditions in which undesired gear shifting occurs. Such undesired gear shifting may take place, for example, if the vehicle is traveling downhill in intermediate gear and the operator lifts his foot from the accelerator pedal. The reduced load on the engine thus tends to cause the automatic transmission to shift upwardly which is contrary to the normal intentions of the operator. The invention thus relates particularly to stepwise operating automatic transmissions, i.e., such transmissions as include gear trains which are selectively coupled into the drive train to provide a number of discrete transmission ratios.
In known stepwise shifting automatic transmission, the shifting points of the transmission depend on the variables of vehicle speed and engine load. Normally, the vehicle speed is detected at the output shaft of the transmission whereas the load condition of the engine is determined from the instantaneous position of the throttle valve or the associated linkage, for example the gas pedal itself.
In hydraulic automatic transmissions, these variables are transformed into pressures which are applied to hydraulic comparators and are used to actuate control plungers which cause the engagement or disengagement of clutches or brake bands so as to block or release the rotation of planetary gears.
In known electronic transmission controls, the operating variables of the vehicle are transformed into electrical signals, in particular voltages, which are then applied to an electronic transmission control circuit containing, for example, multivibrators for controlling solenoid valves which then actuate various gear trains.
In both of the known and above-mentioned transmission control mechanisms, the conditions for shifting from one gear into the next depend on engine load and vehicle speed and can be represented in a load-speed diagram in which the shifting points are depicted as sloping straight lines. Such shifting diagrams determine the operation of the automatic transmission and may include the so-called kick-down shifting, i.e., the forcible downshift by full depression of the accelerator pedal. If, for example, a vehicle is operating at a point of the load-speed diagram which lies above an upshift curve, and if the engine load is reduced, for example, by releasing the gas pedal, the automatic transmission will shift into the next higher gear as determined by the upshift curve. In this manner, the operator of a vehicle equipped with an automatic transmission may control upshift to the next higher gear. However, an upshift of this type can also be annoying because it may not be desired each and every time the gas pedal is released. For example, if the vehicle happens to be traveling downhill or in a curve, the operator normally releases the accelerator at least partially so as to decrease the vehicle speed. In vehicles with manual transmissions, this action has the additional effect of engine braking which can be useful. However, in vehicles with automatic transmissions, the transmission is urged to shift upwardly so that the engine braking of the engine is reduced rather than increased. This type of characteristic in automatic transmissions is not only annoying but may actually constitute hazards in certain traffic situations. A further disadvantage of an automatic transmission occurs when the vehicle is permitted to slow down freely after the accelerator pedal is released. The transmission will sometimes first shift upwardly due to the apparent reduced load and then shift back down as the vehicle speed decreases. This type of operation tends to diminish the driving comfort and also introduces unnecessary wear and tear due to the superfluous shifting events.
In order to counteract the above-described disadvantages, especially in order to utilize engine braking in downhill operation, it is known in the art to provide manual switches which change or defeat the shifting characteristics of the automatic transmission or permit drivers to shift gears by hand. For example, in U.S. Pat. No. 3,732,754, Mori to which German Offenlegungsschrift No. 1 954 783, corresponds there is described an electronic controller for automatic transmissions in which the transducer which detects the accelerator pedal or throttle valve position has a stepped characteristic. The signal pattern of this transducer may be changed by a manual switch or by a switch which is actuated by the gradient of the terrain. Still further, the German Pat. No. 1 155 021 Burchardt describes a control mechanism for automatic gear changing in transmissions, especially for vehicles, in which a manually operated selection valve permits the retention or locking of the transmission in one of the customary driving ranges, i.e., P,R,N, D or L.
It is a disadvantage of these latter known devices that they require a manual engagement of the automatic transmission which runs counter to the general purpose of an automatically operating transmission.
Still further known in the art is an electro-hydraulic controller for an automatic transmission as described in U.S. Pat. No. 3,776,048, to which the German Published Application De-As 2 165 707 corresponds. In that disclosure, a switch located near the accelerator is actuated when the accelerator pedal is released and changes the shifting pattern of the automatic transmission in such a way that, when the accelerator pedal is released and the vehicle is operated at high speed, the transmission shifts from the highest to the next lower gear. The unavoidable downshift may be quite unpleasant at high vehicle speeds and may surprise the driver, thereby initiating a dangerous traffic condition. A further known control system described in the German Offenlegungsschrift No. 2 537 475 provides a switch operated by the accelerator pedal or by the engine exhaust brake which is closed when the accelerator pedal is released and which prevents an upshift or initiates a downshift at high speed. The enforced downshift is also a disadvantage of this apparatus.